The conventional process for producing light olefins from petroleum hydrocarbons is the steam cracking process, the mechanism of which is the steam cracking mechanism of the free radical of hydrocarbons; therefore, the ethylene yield of this process is relatively high. Generally speaking, the mass ratio of propylene to ethylene produced by the steam cracking of naphtha is about 0.43. However, the mass ratio of propylene to ethylene desired in the market is over 0.70. Although the propylene yield can be increased by adequately lowering the reaction severity, the mass ratio of propylene to ethylene is not suitable to exceed 0.65, otherwise, the total yield of light olefins may be decreased and the profit of the process will shrink in consequence. In addition, the feedstock suitable for the steam cracking is light petroleum hydrocarbon such as ethane, propane, butane, natural gas, naphtha or light cycle oil. But the supply amount of light hydrocarbons is limited along with the trend that the crude oil is becoming heavier, so that researchers have to shift their focus to use a wider range of feedstock including heavy petroleum hydrocarbons for producing light olefins.
Over past few years, some patent documents have disclosed the processes for producing light olefins via the catalytic cracking reaction on acidic zeolite by using heavy petroleum hydrocarbons, naphtha, C4-C6 light hydrocarbons and the like as the feedstock. Compared to the steam cracking, the processes of this kind have two superiorities. One is that the mass ratio of propylene to ethylene is high in the gaseous product since the reaction of hydrocarbons on the acidic zeolitic catalyst follows the carbenium ion mechanism. The other is that by utilizing the mature engineering and technology of the fluidized catalytic cracking, the processes of this kind not only can treat light hydrocarbon feedstock, but also can treat heavy hydrocarbon feedstock.
U.S. Pat. No. 4,980,053 disclosed a catalytic cracking process for producing propylene and butylene. This process used a fluidized bed or moving bed reactor and a solid acidic catalyst to carry out the reaction at the reaction temperature of 500-650° C. under the feed weight hourly space velocity of 0.2-20 h−1 with the mass ratio of catalyst to feedstock being 2-12. In its Example 1, the reaction was carried out at 580° C. with a catalyst of ZSM-5 as the active component and kaolin as the support using vacuum gas oil as the feedstock, and it had yields of 5.89 wt % for ethylene, 21.56 wt % for propylene and 15.64 wt % for butylene.
U.S. Pat. No. 6,210,562 disclosed a catalytic pyrolysis process for producing ethylene and propylene. In this process, the preheated heavy petroleum hydrocarbons were brought into contact with a catalyst containing pillared interlayered clay zeolite and/or phosphorous and aluminum or magnesium or calcium modified high silica zeolite having pentasil structure in the presence of high temperature steam in a riser or a downflow transfer-line reactor, and the catalytic pyrolysis reaction was carried out at the temperature of 650-750° C. under the reaction pressure of 1.5-4.0×105 Pa for the reaction time of 0.2-5 seconds with the mass ratio of catalyst to feedstock being 15-40:1 and the mass ratio of steam to feedstock being 0.3-1:1. Both ethylene and propylene were obtained in yields over 18 wt % in this process.
U.S. Pat. No. 6,106,697 disclosed a process for selectively producing C2-C4 olefins by using gas oil or residual oil as the feedstock to carry out the catalytic cracking reaction in a two stage reactor. Gas oil or residual oil was brought into contact with a large pore zeolitic catalyst in the first stage reactor to carry out the catalytic cracking reaction under the conventional catalytic cracking conditions to give products with different boiling ranges including naphtha fraction. The naphtha fraction obtained in the first stage reactor entered the second stage reactor and was brought into contact with a medium pore shape-selective zeolitic catalyst to carry out the further reaction at the reaction temperature of 500-650° C. with the mass ratio of catalyst to feedstock of 4-10:1 under the hydrocarbon partial pressure of 70-280 kPa to give C2-C4 olefins.
To sum up, the main means for increasing yields of ethylene, propylene and butylene in the prior arts are the use of higher reaction temperatures, higher catalyst to oil ratios, and larger amounts of injected steam than those used in the conventional catalytic cracking and the use of catalysts containing medium pore shape-selective zeolite having an average pore diameter less than about 0.7 nanometers. All the above means can intensify the cracking reactions of petroleum hydrocarbons, i.e., all the prior arts attain the objective for increasing yields of ethylene, propylene and butylene by intensifying the formation reaction of ethylene, propylene and butylene. All along, the ordinary skilled in the art generally considered propylene as a stable reaction product under the catalytic cracking reaction conditions, for example, there is a statement on the page 152 of the book “Catalytic Cracking Technology and Engineering (2nd Ed)” by Chen Junwu, China PetroChemical Press, Mar. 1, 2005 that “propylene is a stable product and essentially it does not convert any longer at high feedstock conversion”. Nevertheless, the inventor has surprisingly discovered via laboratory study that propylene possesses a notable reactivity under the reaction conditions for the catalytic conversion of petroleum hydrocarbons to produce light olefins in the presence of an acidic zeolite and it can be converted into other hydrocarbons, hydrogen, and coke rapidly in a large quantities, leading to a reduction of the propylene yield on the contrary.